Thin film transistors have been made heretofore on large glass substrates or plates for use in monitors, flat panel displays, solar cells and the like. The transistors are made by sequential deposition of various films including amorphous silicon, both doped and intrinsic, silicon oxide, silicon nitride and the like in a vacuum chamber. Thin films for transistors can be deposited by chemical vapor deposition (hereinafter CVD) for example.
CVD is a comparatively high temperature process requiring that the substrates withstand temperatures on the order of 350.degree.-400.degree. C. CVD film processing has found widespread use in the manufacture of integrated circuits in silicon wafers. Silicon is a conductive material and it can be heated and cooled quite rapidly without breaking or warping the wafer. However, glass is a dielectric material that is very brittle and is subject to warping or cracking when cooled or heated too rapidly. Thus great care must be taken to adjust the rate of heating or cooling of large area glass substrates to avoid thermal stress and resulting damage.
Presently, equipment used for depositing thin films on glass substrates, usually mount several glass substrates on a single large metal carrier plate for processing. The vacuum processing chambers are large, and, because of the thermal mass of the carrier plate and of the glass substrate, heating to processing temperatures requires a lengthy period of time (greater than 20 minutes).
Present day thin film equipment for the semiconductor industry is increasingly moving toward single substrate processing because the individual processing chambers can be made much smaller, processing can be better controlled, and the need for a carrier plate can be eliminated. Further, modern semiconductor equipment is increasingly moving toward systems that can carry out more than one process step on a wafer without removing the wafer from a vacuum environment, which results in reduced particulates which in turn results in fewer rejected devices. Such systems include a central robotic chamber connected to various processing chambers, such as the Applied Materials 5000 series processing systems described in U.S. Pat. No. 4,951,601 to Maydan et al.
It would be desirable to process large glass substrates in similar fashion, depositing multilayer thin films for forming transistors in a single vacuum system using single substrate film processing chambers. Advantages are that smaller processing chambers could be utilized, the vacuum system would be a cleaner system, eliminating or greatly reducing particulate contamination, and the processing could be more controllable and uniform.
However, because of the lengthy period of time required to heat up and cool down the temperature of the glass substrates, e.g., about 5 minutes are required both to heat a large area glass substrate to about 400.degree. C. and to cool it back to room temperature after processing to avoid damage or warpage to the substrate, several glass substrates must be heated or cooled at the same time to improve the throughput of the system.